Bimolecular Excited-State Proton-Coupled Electron Transfer within Encounter Complexes

Bimolecular excited-state proton-coupled electron transfer (PCET*) was observed for reaction of the triplet MLCT state of [(dpab)2Ru(4,4′-dhbpy)]2+ (dpab = 4,4′-di(n-propyl)amido-2,2′-bipyridine, 4,4′-dhbpy = 4,4′-dihydroxy-2,2′-bipyridine) with N-methyl-4,4′-bipyridinium (MQ+) and N-benzyl-4,4′-bipyridinium (BMQ+) in dry acetonitrile solutions. The PCET* reaction products, the oxidized and deprotonated Ru complex, and the reduced protonated MQ+ can be distinguished from the excited state electron transfer (ET*) and the excited state proton transfer (PT*) products by the difference in the visible absorption spectrum of the species emerging from the encounter complex. The observed behavior differs from that of reaction of the MLCT state of [(bpy)2Ru(4,4′-dhbpy)]2+ (bpy = 2,2′-bipyridine) with MQ+, where initial ET* is followed by diffusion-limited proton transfer from the coordinated 4,4′-dhbpy to MQ0. The difference in observed behavior can be rationalized based on changes in the free energies of ET* and PT*. Substitution of bpy with dpab results in the ET* process becoming significantly more endergonic and the PT* reaction becoming somewhat less endergonic.


PT* Reaction of Photoexcited [(dpab)2Ru(4,4'-dhbpy)] 2+ and 3-acetylpyridine.
The possibility of PT* was investigated for the reaction between [(dpab)2Ru(4,4'-dhbpy)] 2+ and 3-acetylpyridine. 3-acetylpyridine, with a pKa of 10.7 in acetonitrile, was chosen as a model base for MQ + , being slightly more basic than MQ + (pKa of 10.3). 16 Although the free energy for the proton exchange reaction (+2.3 kcal/mol) is similar to what is expected of MQ + , electrostatic factors must also be considered. Because MQ + is a cation, there is a thermodynamic penalty for bringing together the likecharged species, however, in considering the overall proton transfer reaction, the separation of species after reaction is also important. Taking both formation of the encounter complex and separation of the proton transfer products into consideration, the net change in charge is zero, and therefore its contribution to the free energy for proton transfer is likely to be negligible. This is not the case for the model PT study with 3-acetylpyridine. As such, the free energy for excited-state proton transfer must include the calculation of the energy to bring together the dicationic Ru complex and the uncharged 3-acetylpyridine, and for the separation of the two monocationic species. An electrostatic work term can be included in the calculation of the free energy for proton transfer much like in photoinduced electron transfer reactions.

Equation 4
In the expressions above, WP and WR are the work terms for product separation and formation of the encounter complex respectively, where z is the charge of the species, e is the elementary charge, ε0 is the permittivity of vacuum (8.854 x 10 -12 C 2 J -1 m -1 ), εr is the relative medium static permittivity (dielectric constant), and a is the distant between the two species). In the reaction between [(dpab)2Ru(4,4'-dhbpy)] 2+ and 3-acetylpyridine, WR is zero, and so only the term for product separation needs to be included. The distance between the metal complex and 3-acetylpyridine was calculated from the collision radii for the two species using the molecular mechanics optimized structures. Taking this into account, the calculated work term is -0.82 kcal/mol, making the free energy for proton transfer +1.5 kcal/mol.
The reaction of [(dpab)2Ru(4,4'-dhbpy)] 2+ and 3-acetylpyridine was explored with transient absorption spectroscopy. 3-acetlypyridine and its conjugate acid do not absorb in the visible region of the spectrum, so the PT products are expected to match the simulated spectrum shown in figure 5. Stern-Volmer quenching analysis showed that PT* is slow for this system ( kq = 1.3 x 10 7 M -1 s -1 ) , so concentrations of 3-acetylpyridine as high as 500 mM were used in TA analysis.. The transient absorption spectrum shown in Figure 7 shows the prominent absorption features that allow PT* products to be distinguished from ET* and PCET* products. The absorbance at around 530 nm is a distinguishing feature for the proton transfer products, along with the lower molar absorptivity absorption at 400 nm.
The proton transfer to 3acetylpyridine is slightly less endergonic than that to MQ + . Thus, PT* could be observed between [(dpab)2Ru(4,4'-dhbpy)] 2+ and MQ + and it would be revealed by the TA spectrum.

Equations Used for Determination of Free Energies for PT*, ET* and PCET*
A. PT*: For calculating the free energy for proton transfer, the calculation is made by taking the difference in pKa value between the chromophore and the base, this gives the equilibrium constant for the proton transfer reaction, which can then be used to calculate the Gibbs free energy for proton transfer. This can be done for the ground state, excited state, and the one-electron oxidized state of the chromophore. For the proton transfer reaction, the work term for bringing the two charged species will be zero. Equations 5-7 show the calculation of the free energy for proton transfer including the reactant and product work terms. In these equations, R is the ideal gas constant, T is the temperature, ΔpKa is the difference in pKa between the chromophore in either the ground state, excited state or one electron oxidized forms, and the quencher. NA is Avogadro's number, WP is the work terms for product separation, WR is the work term for bringing together the reactants, ε0 is the permittivity of vacuum constant, a is the distance between the two species, z is the charge on each compound, and e is the elementary charge.
Because the reactants will go from a 2+/1+ combination to products with a 1+/2+ charge combination for the pure proton transfer in either the ground or excited state, the work term will end up being zero. For proton transfer following electron transfer, the work term cannot be omitted, as Wp will be nonzero, while Wr will be zero, assuming the separation distance remains constant. The same equations for work can be applied to the electron transfer reaction and proton coupled electron transfer reactions, as will be discussed further below.
B. ET*: The free energy for the electron transfer can likewise be calculated using the redox potentials for the complexes and the quenchers. ΔGET can be calculated for the ground state and excited state electron transfer reactions using equation 8. This again includes the work term for bringing the charged species together along with the work required for charge separation. The pair will go from 2+/1+ for formation of the precursor complex to 3+/0 following electron transfer. The distance between the complex and the quencher has been estimated using a value of 6 Å for the ruthenium complex based on [Ru(bpy)3] 2 + and the collision radius for the molecular mechanics optimized structure of the quenchers. From gas phase molecular mechanics calculations, the collision radii of MQ + and BMQ + were found to be 3.5 Å and 4.5 Å, respectively. In equation 8, F is the Faraday constant, n is the number of electrons being transferred, and E 0 (Ru III/II ) and E 0 (MQ +/0 ) are the reduction potentials for the ruthenium(III) complex and bipyridinium quencher.
For calculation of the free energy for excited state electron transfer, the oxidation potential of the excited state of the ruthenium complex (E 0 (Ru III/II )-Eem) is substituted in for E 0 (Ru III/II )).
C. PCET*: For calculation of the free energy for proton-coupled electron transfer, two methods can be used, and should yield the same result within a margin of error attributable to the error associated with individual experimental values. The first method involves taking the sum of the free energy for PT*, followed by ET* from the deprotonated complex to the protonated quencher. The second method involves the sum of free energies for ET* and ground state proton transfer from the Ru III to the reduced quencher. The reason for this is that excited state electron transfer deactivates the excited state, such that any proton transfer subsequently should occur from the ground state potential surface. The more reliable estimates come from the first approach, since determination of the pKa values for the Ru III hydroxybipyridine complex and the reduced MQ + (MQ 0 ) have large margins of error.